Image display apparatus

ABSTRACT

An image display apparatus includes a light source device including a light source unit; a scanning optical system including an image forming unit on which an intermediate image is formed by light from the light source unit; and a virtual image optical system configured to guide light of the intermediate image by using a reflecting mirror and a curved transmissive reflection member. The scanning optical system includes an optical scanning unit configured to scan the light from the light source unit in a main scanning direction and a sub-scanning direction of the image forming unit. The image forming unit is a transmissive member curved with a convex surface toward the reflecting mirror.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.15/496,868, filed Apr. 25, 2017, is a continuation of U.S. applicationSer. No. 15/066,448, filed Mar. 10, 2016, now U.S. Pat. No. 9,864,195,and claims priority to and incorporates by reference the entire contentsof Japanese Patent Application No. 2015-047887 filed in Japan on Mar.11, 2015.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an image display apparatus.

2. Description of the Related Art

An image display apparatus called head-up display to be mounted on amoving body (object), such as an automobile, aircraft, and a ship, hasbeen known. A head-up display is an image display apparatus fordisplaying various types of information related to operations of themoving body in a manner easily visible to the user. A head-up displayincludes an optical element called combiner, and projects and displaysimage light for displaying the information upon the combiner. The imagelight projected on the combiner presents display related to theinformation to the user in a manner such that the display can bevisually observed as a virtual image farther than the physical positionof the combiner (on a far side from the user). A front windshield of anautomobile and the like may be used as the combiner. A differenttransmissive reflection member may be used as the combiner.

To improve the viewability of the information displayed by the head-updisplay, distortion of the virtual image needs to be reduced. Thevirtual image is generated from an intermediate image formed by an imageforming unit. There is known a head-up display in which a free-formsurface lens is arranged in the preceding stage of the image formingunit to correct distortion caused by the image forming unit (forexample, see Japanese Patent No. 5370427).

The head-up display is arranged in front of the user (driver) of themoving body. If the moving body is an automobile, the head-up display isaccommodated in the dashboard. What is desired of the head-up display isthen to satisfy a demand for further miniaturization and improve theviewability of information.

Like the example of Japanese Patent No. 5370427, an optical systemincluding an image forming unit for generating an intermediate image mayinclude a correction optical element for correcting distortion and/orresolution of the intermediate image. This, however, goes against thedemand for miniaturization. In addition, the need for the correctionoptical element results in an increase in cost.

Therefore, there is a need to provide an image display apparatus ofwhich size and cost can be reduced while maintaining viewability(quality) of a virtual image.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve theproblems in the conventional technology.

According to an embodiment, there is provided an image display apparatusthat includes a light source device including a light source unit; ascanning optical system including an image forming unit on which anintermediate image is formed by light from the light source unit; and avirtual image optical system configured to guide light of theintermediate image by using a reflecting mirror and a curvedtransmissive reflection member. The scanning optical system includes anoptical scanning unit configured to scan the light from the light sourceunit in a main scanning direction and a sub-scanning direction of theimage forming unit. The image forming unit is a transmissive membercurved with a convex surface toward the reflecting mirror.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram illustrating an example of a head-updisplay that is an embodiment of an image display apparatus according tothe present invention;

FIG. 2 is a configuration diagram illustrating an example of a lightsource device included in the head-up display;

FIG. 3A is a plan view illustrating an example of a scanning opticalsystem included in the head-up display;

FIG. 3B is a side view illustrating the example of the scanning opticalsystem;

FIG. 4A is an optical path diagram in a direction of a main scanningsection in Example 1 of the scanning optical system including a flatto-be-scanned surface element;

FIG. 4B is an optical path diagram in a direction of a main scanningsection in Example 1 of the scanning optical system including a curvedto-be-scanned surface element;

FIG. 5A is a graph illustrating an example of a correlation between aposition and a beam spot diameter on the flat to-be-scanned surfaceelement in Example 1 of the scanning optical system;

FIG. 5B is a graph illustrating an example of a correlation between aposition and a beam spot diameter on the curved to-be-scanned surfaceelement in Example 1 of the scanning optical system;

FIG. 6A is a graph illustrating an example of a correlation in the mainscanning direction between the beam spot diameter and an arrivalposition of light on the flat to-be-scanned surface element when movedin Example 1;

FIG. 6B is a graph illustrating an example of a correlation in thesub-scanning direction between the beam spot diameter and an arrivalposition of light on the flat to-be-scanned surface element when movedin Example 1;

FIG. 6C is a graph illustrating an example of a correlation in the mainscanning direction between the beam spot diameter and an arrivalposition of light on the curved to-be-scanned surface element when movedin Example 1;

FIG. 6D is a graph illustrating an example of a correlation in thesub-scanning direction between the beam spot diameter and an arrivalposition of light on the curved to-be-scanned surface element when movedin Example 1;

FIG. 7A is a diagram illustrating arrival positions of light on the flatto-be-scanned surface element in Example 1;

FIG. 7B is a diagram illustrating arrival positions of light on thecurved to-be-scanned surface element in Example 1;

FIG. 8A is an optical path diagram in a direction of a main scanningsection in Example 2 of the scanning optical system including a flatto-be-scanned surface element;

FIG. 8B is an optical path diagram in a direction of a main scanningsection in Example 2 of the scanning optical system including a curvedto-be-scanned surface element;

FIG. 9A is a graph illustrating an example of a correlation between aposition and a beam spot diameter on the flat to-be-scanned surfaceelement in Example 2 of the scanning optical system;

FIG. 9B is a graph illustrating an example of a correlation between aposition and a beam spot diameter on the curved to-be-scanned surfaceelement in Example 2 of the scanning optical system;

FIG. 10A is a graph illustrating an example of a correlation in the mainscanning direction between the beam spot diameter and an arrivalposition of light on the flat to-be-scanned surface element when movedin Example 2;

FIG. 10B is a graph illustrating an example of a correlation in thesub-scanning direction between the beam spot diameter and an arrivalposition of light on the flat to-be-scanned surface element when movedin Example 2;

FIG. 10C is a graph illustrating an example of a correlation in the mainscanning direction between the beam spot diameter and an arrivalposition of light on the curved to-be-scanned surface element when movedin Example 2;

FIG. 10D is a graph illustrating an example of a correlation in thesub-scanning direction between the beam spot diameter and an arrivalposition of light on the curved to-be-scanned surface element when movedin Example 2;

FIG. 11A is a diagram illustrating arrival positions of light on theflat to-be-scanned surface element in Example 2;

FIG. 11B is a diagram illustrating arrival positions of light on thecurved to-be-scanned surface element in Example 2; and

FIG. 12 is a perspective view of the to-be-scanned surface elementincluded in the scanning optical system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Head-Up Display

A head-up display (hereinafter, referred to as “HUD”) which is anembodiment of an image display apparatus according to the presentinvention will be described below with reference to the drawings. Asillustrated in FIG. 1, a HUD 1000 includes a light source device 100, ascanning optical system 200 which is a first optical system, and avirtual image optical system 300 which is a second optical system.

The HUD 1000 is an image display apparatus which is mounted on an object(moving body such as an automobile, aircraft, and a ship) and displaysinformation about an operation and control of the object in an easilyvisible manner. The following description will be given by using anexample where the HUD 1000 is mounted on an automobile.

The HUD 1000 forms an intermediate image including information to bedisplayed in the scanning optical image 200. The intermediate image ismagnified and projected by the virtual image optical system 300 to bevisually observable by a user 11 as a magnified virtual image 12. Thescanning optical system 200 forms the intermediate image by using lightemitted from the light source device 100.

The virtual image optical system 300 according to the present embodimentuses a front windshield 10 of the automobile as a combiner 302. Insteadof the front windshield 10, another transmissive reflection member maybe used as the combiner 302. In the following description, the target onwhich the virtual image optical system 300 projects the intermediateimage will be referred to as either “combiner 302” or “front windshield10”.

The front windshield 10 of the automobile is tilted in a verticaldirection in the field of view of the user 11. The upper side of thefront windshield 10 is closer to the user 11. The lower side is fartherfrom the user 11. The front windshield 10 is curved in a horizontaldirection in the field of view of the user 11. If the automobile has aright-hand drive configuration, the front windshield 10 is curved torecede from the user from the left side to the middle area of the fieldof view of the user 11, and approach the user 11 from the middle area tothe right side.

If an intermediate image formed by the scanning optical system 200 isprojected on the combiner 302, the intermediate image is visible to theuser 11 as the magnified virtual image 12 in a position farther from thephysical position of the combiner 302. The magnified virtual image 12displays information about an operating state and the like of theautomobile (navigation information such as moving speed, traveldistance, and a destination display).

A HUD 1000 of a type that uses the front windshield 10 as the combiner302 to project the intermediate image on will be referred to as a frontwindshield projection type. A HUD 1000 of a type that uses atransmissive reflection member other than the front windshield 10 willbe referred to as a combiner projection type. There is no difference indisplayable information between the two types. A front windshieldprojection type is preferred in terms of designability of a cabin spaceinside the automobile, the annoying presence of an object (combiner 302)other than the front windshield 10 in the field of view of the user 11,etc.

In the case of the front windshield projection type, the optical systemthat generates the intermediate image (scanning optical system 200) isusually embedded in the dashboard of the vehicle. The point of view ofthe user 11 simply represents a referential viewpoint position(reference eye point). The range of the point of view of the user 11 isequivalent to or narrower than the eye range of drivers for automobiles.The driver eye range is defined in, for example, Japanese IndustrialStandards (JIS) D 0021.

A three-dimensional orthogonal coordinate system used in the descriptionof the present embodiment will be described with reference to FIG. 1. InFIG. 1, the direction from the user 11 to the magnified virtual image12, or equivalently, the direction of the field of view of the user 11corresponds to the forward traveling direction of the vehicle on whichthe HUD 1000 is mounted.

As illustrated in FIG. 1, the direction of the field of view of the user11 will be referred to as a Z-axis direction, the forward direction −Zdirection, and the backward direction +Z direction. A horizontaldirection in the field of view of the user 11 will be referred to as anX-axis direction. The right-hand direction to the user 11 (directiontoward the far side of the plane of FIG. 1) will be referred to as a +Xdirection, and the left-hand direction (direction toward the near sideof the plane of FIG. 1) −X direction. A vertical direction in the fieldof view of the user 11 will be referred to as a Y-axis direction. Theupward direction to the user 11 will be referred to as a +Y direction,and the downward direction −Y direction. In other words, the widthdirection of the automobile will be referred to as an X-axis direction,the height direction Y-axis direction, and the length direction Z-axisdirection.

An overall configuration of the HUD 1000 will be described. Asillustrated in FIG. 1, the intermediate image (image light) from amagnifying concave mirror 301 constituting the virtual optical system300 is incident on the combiner 302. The center of the incident area ofthe intermediate image will be referred to as an “incident area center303”. As seen in the X-axis direction, a tangential plane at theincident area center 303 tilts with respect to a first virtual axis 304which connects the point of view of the user 11 and the center of themagnified virtual image 12 (virtual image center 305). As seen in theY-axis direction, the tangential plane at the incident area center 303tilts with respect to the first virtual axis 304. The first virtual axis304 is an axis passing the incident area center 303.

The center of the reflecting surface of the magnifying concave mirror301 will be referred to as a reflecting surface center 307. Thereflecting surface center 307 is the center of an effective reflectingarea of the magnifying concave mirror 301 and the center of a light beamthat is incident on the virtual optical system 300 from the scanningoptical system 200.

Assume that a second virtual axis 306 connects the center of theintermediate image formed by the scanning optical system 200 (the centerof a to-be-scanned surface element 202 to be described later) and thereflecting surface center 307. As illustrated in FIG. 1, the secondvirtual axis 306, when seen in the X-axis direction, tilts with respectto the first virtual axis 304. When seen in the Y-axis direction, thesecond virtual axis 306 tilts with respect to the first virtual axis304. A “center of a surface to be scanned” shall refer to the center ofan effective scanning area of the to-be-scanned surface element 202 tobe described later.

Virtual Image Optical System

A detailed configuration of the virtual optical system 300 will bedescribed. As illustrated in FIG. 1, the virtual image optical system300 includes the magnifying concave mirror 301 and the combiner 302. Thecombiner 302 has already been described. A “two-dimensional color image”formed by the to-be-scanned surface element 202 to be described later isdelivered to the magnifying concave mirror 301 as an intermediate image.The intermediate image is delivered in the form of light in units ofpixels of image information (light corresponding to each pixel). Themagnifying concave mirror 301 reflects the intermediate image toward thecombiner 302.

Light Source Device

Next, components of the HUD 1000 will be described. As illustrated inFIG. 2, the light source device 100 emits an image display beam 101 tobe used to form the magnified virtual image 12 which is a color image.The image display beam 101 is a light beam obtained by combining beamsin three colors, red (hereinafter, denoted as “R”), green (hereinafter,denoted as “G”), and blue (hereinafter, denoted as “B”).

The light source device 100 includes first to third independent lightsources 110, 120, and 130. The first light source 110 emits red laserlight. The second light source 120 emits green laser light. The thirdlight source 130 emits blue laser light. The first, second, and thirdlight sources 110, 120, and 130 are semiconductor laser devices. Laserdiodes (LDs) called edge emitting lasers or vertical cavity surfaceemitting lasers (VCSELs) may be used. LED devices may be used as thelight sources instead of semiconductor laser devices.

The light source device 100 includes a first collimator lens 111, asecond collimator lens 121, and a third collimator lens 131 whichsuppress the divergence of the light emitted from the respective lightsources.

The light source device 100 includes a first aperture 112, a secondaperture 122, and a third aperture 132 which correspond to the lightpassed through the respective collimator lenses. The apertures 112, 122,and 132 regulate the light beam diameters of the light to shape thelight beams. The light source device 100 further includes a beamcombining prism 140 which combines the shaped light beams of therespective colors and emits the image display beam 101, and a condenserlens 150.

The beam combining prism 140 includes a first dichroic film 141 whichtransmits red light and reflects green light, and a second dichroic film142 which transmits red light and green light and reflects blue light.

The red light emitted from the first light source 110 is incident on thebeam combining prism 140 via the first collimator lens 111 and the firstaperture 112. The red light incident on the beam combining prism 140travels straight through the first dichroic film 141.

The green light emitted from the second light source 120 is incident onthe beam combining prism 140 via the second collimator lens 121 and thesecond aperture 122. The green light incident on the beam combiningprism 140 is reflected by the first dichroic film 141 and guided in thesame direction as the red light (toward the second dichroic film 142).

The blue light emitted from the third light source 130 is incident onthe beam combining prism 140 via the third collimator lens 131 and thethird aperture 132. The blue light incident on the beam combining prism140 is reflected by the second dichroic film 142 in the same directionas the red light and the green light.

As described above, the red light and the green light passed through thesecond dichroic film 142 and the blue light reflected by the seconddichroic film 142 are emitted from the beam combining prism 140 in thesame direction. The laser light emitted from the beam combining prism140 is one laser light beam into which the red light, the green light,and the blue light are combined. The laser light beam is converted intothe image display beam 101 by the condenser lens 150.

The condenser lens 150 is an optical element that guides the imagedisplay beam 101, which is converging light, to a two-dimensionaldeflection element 201 (see FIGS. 3A and 3B) serving as an opticalscanning unit to be described below. The condenser lens 150 preferablyhas a focal length f of greater than 150 mm at a wavelength of 587.65 nm(reference wavelength).

The laser light beams of the respective colors R, G, and B constitutingthe image display beam 101 are modulated in intensity according to asignal or data related to the “two-dimensional color image” to bedisplayed. The intensity modulation of the laser light beams may beimplemented by directly modulating the semiconductor lasers of therespective colors (direct modulation method) or by modulating the laserlight beams emitted from the semiconductor lasers of the respectivecolors (external modulation method).

In other words, the light sources emit the laser light of the respectivecolors of which emission intensity is modulated according to imagesignals of the respective color components R, G, and B by driving meansdriving the light sources.

Scanning Optical System

Next, the scanning optical system 200 will be described in detail. Theto-be-scanned surface element 202 included in the scanning opticalsystem 200 will initially be described. FIG. 12 is a perspective view ofthe to-be-scanned surface element 202 as seen obliquely from above theincident direction of the image display beam 101. As illustrated in FIG.12, with the X direction as a longitudinal direction and the Y directionas a transverse direction, the to-be-scanned surface element 202 has alongitudinally-curved cylindrical shape. A convex portion correspondingto the top of the curved portion of the to-be-scanned surface element202 is directed in the −Z direction. The to-be-scanned surface element202 may also be curved in the transverse direction. If the to-be-scannedsurface element 202 is also curved in the transverse direction, theconvex surface is directed in the −Z direction.

As illustrated in FIGS. 3A and 3B, the scanning optical system 200including the to-be-scanned surface element 202 described above includesthe two-dimensional deflection element 201 serving as optical scanningmeans, and the to-be-scanned surface element 202 serving as an imageforming unit.

FIG. 3A is an optical layout diagram illustrating the scanning opticalsystem 200 on the XZ plane as seen in the +Y direction from the −Y side.In other words, FIG. 3A is a plan view of the scanning optical system200 as seen from below.

FIG. 3B is an optical layout diagram illustrating the scanning opticalsystem 200 on the YZ plane as seen in the +X direction from the −X side.In other words, FIG. 3B is a side view of the scanning optical system200 as seen from the left.

As illustrated in FIG. 3A, the to-be-scanned surface element 202 iscurved with the convex surface toward the magnifying convex mirror 301at least on the XZ plane. As has been described, the X direction is thelongitudinal direction of the to-be-scanned surface element 202. Thislongitudinal direction coincides with that of the magnified virtualimage 12.

As illustrated in FIG. 3B, the to-be-scanned surface element 202 may becurved with the convex surface toward the magnifying concave mirror 301on the YZ plane. As has been described, the Y direction is thetransverse direction of the to-be-scanned surface element 202. Thistransverse direction coincides with that of the magnified virtual image12.

The to-be-scanned surface element 202 is a transmissive member whichshows the intermediate image to the side of the magnifying concavemirror 301 when optically scanned by the two-dimensional deflectionelement 201.

As illustrated in FIGS. 3A and 3B, the scanning optical system 200 doesnot include a condenser element for giving the image display beam 101 acondensing effect or diverging effect between the two-dimensionaldeflection element 201 and the to-be-scanned surface element 202. TheHUD 1000 thus allows a cost reduction while improving robustness.

The two-dimensional deflection element 201 serving as the opticalscanning unit is an element that two-dimensionally deflects the imagedisplay beam 101 emitted from the light source device 100. Thetwo-dimensional deflection element 201 is an assembly of micromirrorsconfigured to rock by using two mutually orthogonal shafts. Thetwo-dimensional deflection element 201 is a microelectromechanicalsystem (MEMS) produced as a micro rocking mirror element bysemiconductor processes etc. The structure of the MEMS used as thetwo-dimensional deflection element 201 is not limited to this example.For example, two micromirrors may be arranged on one shaft and the twomicromirrors may be configured to rock about the one shaft in mutuallyorthogonal directions.

The image display beam 101 is incident on the to-be-scanned surfaceelement 202 according to a deflection operation of the two-dimensionaldeflection element 201. The to-be-scanned surface element 202 istwo-dimensionally scanned by the image display beam 101 in a mainscanning direction and a sub-scanning direction. More specifically, forexample, raster scan is performed so that the to-be-scanned surface 202is scanned at high speed in the main scanning direction and scanned atlow speed in the sub-scanning direction. The two-dimensional scanning ofthe to-be-scanned surface element 202 forms an intermediate image. Theintermediate image formed here is a “two-dimensional color image”. Whilethe present embodiment is described by assuming a color image, amonochrome image may be formed on the to-be-scanned surface element 202.

What is displayed on the to-be-scanned surface element 202 at eachmoment is “only a pixel that is irradiated with the image display beam101 at that moment”. The “two-dimensional color image” is thus formed asa “set of pixels displayed at respective moments”, resulting from thetwo-dimensional scanning of the image display beam 101.

The to-be-scanned surface element 202 includes small convex lenses. Theintermediate image formed on the to-be-scanned surface element 202 bythe image display beam 101 appears on the side of the virtual imageoptical system 300 as magnified by the small convex lens structure. Thismagnified intermediate image is reflected by the magnifying convexmirror 301 and projected on the combiner 302. The combiner 302 reflectsthe projected image toward the user 11. This image is focused on theretinas of the user 11 and visually observed as the magnified virtualimage 12. With such a configuration, the user 11 can visually observethe magnified virtual image 12 with reliability even if the user 11makes some head movement (moves the point of view).

The to-be-scanned surface element 202 is not limited to the small convexlens structure (microlens array). A diffusion plate, a translucentscreen, a reflection screen, or the like may be used. In the presentembodiment, the to-be-scanned surface element 202 which is a microlensarray is assumed to include a plurality of two-dimensionally arrangedmicrolenses. Instead, a plurality of one-dimensionally arrangedmicrolenses or three-dimensionally arranged microlenses may be used.

EXAMPLE 1

Next, an embodiment of the scanning optical system 200 will bedescribed. Table 1 below lists an example of specifications of the HUD1000 according to Example 1 and dimensions of the to-be-scanned surfaceelement 202 serving as the image forming unit.

TABLE 1 VIRTUAL IMAGE POSITION (DISTANCE 6 m FROM OBSERVER 11 TOMAGNIFIED VIRTUAL IMAGE 12) ANGLE OF VIEW (ANGLE AT WHICH 7° × 3°OBSERVER 11 VIEWS MAGNIFIED VIRTUAL IMAGE 12) SIZE OF IMAGE FORMING UNIT(TO-BE- 33 mm × 16 mm SCANNED SURFACE ELEMENT 202)

Table 2 below lists an example of the specifications of thetwo-dimensional deflection element 201 (MEMS) which is the opticalscanning unit. The tilt angle is of the micromirrors included in thetwo-dimensional deflection element 201.

TABLE 2 MAIN SCANNING SUB-SCANNING DIRECTION DIRECTION TILT ANGLE (°)16.7 9.3 EFFECTIVE SIZE (mm) 0.75 0.98

Table 3 below lists an example of data related to optical elementsincluded in the light source device 100 and the scanning optical system200. “Surface number” in Table 3 is assigned with the light emittingpoints of the first, second, and third light sources 110, 120, and 130included in the light source device 100 as the “zeroth surface”. Thelight incident side of the to-be-scanned surface element 202 is theninth surface. The first to eighth surfaces refer to surfaces that givean optically effect to the light emitted from the light source units.Examples of the optical effect include convergence and divergence.

TABLE 3 Y RADIUS OF X RADIUS OF SURFACE SURFACE CURVATURE Ry CURVATURERx DISTANCE NUMBER (mm) (mm) (mm) MATERIAL REMARKS 0 1.00E+18 1.00E+182.46 LIGHT EMITTING POINTS OF 1ST, 2ND, AND 3RD LIGHT SOURCE UNITS 110,120, AND 130 1 1.00E+18 1.00E+18 0.25 SBSL7 2 1.00E+18 1.00E+18 5.26 31.00E+18 1.00E+18 5.00 DZK3 1ST COLLIMATOR 4 −6.428  −6.428  0.20 LENS111 2ND COLLIMATOR LENS 121 3RD COLLIMATOR LENS 131 5 1.00E+18 1.00E+1827.83 1ST APERTURE 112 2ND APERTURE 122 3RD APERTURE 132 6 4.200 4.2003.00 LLAM60 CONDENSER LENS 150 7 3.010 3.010 56.00 8 1.00E+18 1.00E+18−72.03 TWO-DIMENSIONAL DEFLECTION ELEMENT 201 9 1.00E+18 44.8   0.00TO-BE-SCANNED SURFACE ELEMENT 202

The fifth surface listed in Table 3 corresponds to the aperture surfacesof the respective first, second, and third apertures 112, 122, and 132.The opening areas of the apertures are defined by the sizes of theaperture surfaces in the main scanning direction and the sub-scanningdirection. The apertures have opening areas of different sizes.

The aperture surface of the first aperture 112 corresponding to the redlight source has a size of 2.08 mm in the main scanning direction and3.04 mm in the sub-scanning direction. The aperture surfaces of thesecond aperture 122 corresponding to the green light source and thethird aperture 132 corresponding to the blue light source both have asize of 2.0 mm in the main scanning direction and 2.4 mm in thesub-scanning direction. That is, the first aperture 112 corresponding tothe first light source unit which is the red light source has an openingarea greater than that of both the second and third apertures 122 and132. This can increase the input efficiency of light from the red lightsource.

As listed in Table 3, the to-be-scanned surface element 202 has a Yradius of curvature Ry (1.00E+18) and an X radius of curvature Rx (44.8)which have a relationship of |Rx|<|Ry|. The Y radius of curvature Ry ofthe to-be-scanned surface element 202 refers to the radius of curvatureat the center of the to-be-scanned surface element 202 in the transversedirection (Y direction). The X radius of curvature Rx of theto-be-scanned surface element 202 refers to the radius of curvature atthe center of the to-be-scanned surface element 202 in the longitudinaldirection (X direction).

The radius of curvature at the center of the to-be-scanned surfaceelement 202 in the transverse direction (Y radius of curvature Ry) is1.00E+18, which means a substantially flat surface. The radius ofcurvature at the center of the to-be-scanned surface element 202 in thelongitudinal direction (X radius of curvature Rx) is 44.8. Theto-be-scanned surface element 202 thus has a longitudinally-curvedcylindrical shape.

As listed in Table 3, the condenser lens 150 is an optical element thatguides converging light to the two-dimensional deflection element 201which is the optical scanning unit. The condenser lens 150 has a focallength f of greater than 150 mm with respect to light having awavelength of 587.56 nm. This can reduce the beam spot diameter on theto-be-scanned surface element 202 to form an intermediate image havinghigh resolution.

Next, optical performance of the HUD 1000 including the scanning opticalsystem 200 according to Example 1 will be described. FIGS. 4A and 4Billustrate examples of an optical path diagram of the to-be-scannedsurface element 202 in the direction of a main scanning section. In thefollowing description of Example 1, the optical performance of the HUD1000 will be described by using a case where the to-be-scanned surfaceelement 202 is assumed to have a flat shape as illustrated in FIG. 4A asa comparative example. The following specifications related to theoptical performance were calculated by calculator simulation.

FIG. 4A is an optical path diagram of the comparative example in whichthe to-be-scanned surface element 202 is assumed to be flat in thelongitudinal direction. FIG. 4B is an optical path diagram of theto-be-scanned surface element 202 that has the curved shape in the mainscanning direction as has been described.

Next, a correlation between an arrival position of the image displaybeam 101 on the to-be-scanned surface element 202 according to Example 1and the size of the beam spot diameter will be described with referenceto FIGS. 5A and 5B. The horizontal axes of the graphs of FIGS. 5A and 5Bindicate the arrival position of light on the to-be-scanned surfaceelement 202 when the micromirrors of the two-dimensional deflectionelement 202 are set to the angles in the main scanning direction and thesub-scanning direction corresponding to “number” in the following Table4. The angles in Table 4 indicate those of the micromirrors with respectto the optical axis of the scanning optical system 200. The verticalaxes of the graphs of FIGS. 5A and 5B indicate the size of the beam spotdiameter.

FIG. 5A illustrates the graph of the comparative example where theto-be-scanned surface element 202 is assumed to be flat. FIG. 5Billustrates the graph of the to-be-scanned surface element 202. As isclear from a comparison between the graphs of FIGS. 5A and 5B, if theto-be-scanned surface element 202 has a longitudinally curved shape, thesize of the beam spot diameter at each position on the to-be-scannedsurface element 202 varies less than when the to-be-scanned surfaceelement 202 is a flat one.

TABLE 4 MAIN SCANNING SUB-SCANNING NUMBER DIRECTION (°) DIRECTION (°) 1−10.2 7.8 2 −10.2 5.3 3 −10.2 2.7 4 −10.2 0.0 5 −10.2 −2.7 6 −10.2 −5.37 −10.2 −7.8 8 −13.7 7.8 9 −13.7 5.3 10 −13.7 2.7 11 −13.7 0.0 12 −13.7−2.7 13 −13.7 −5.3 14 −13.7 −7.8 15 −17.2 7.8 16 −17.2 5.3 17 −17.2 2.718 −17.2 0.0 19 −17.2 −2.7 20 −17.2 −5.3 21 −17.2 −7.8 22 −11.8 7.8 23−11.8 5.3 24 −11.8 2.7 25 −11.8 0.0 26 −11.8 −2.7 27 −11.8 −5.3 28 −11.8−7.8 29 −15.6 7.8 30 −15.6 5.3 31 −15.6 2.7 32 −15.6 0.0 33 −15.6 −2.734 −15.6 −5.3 35 −15.6 −7.8

Next, variations of the beam spot diameter in each arrival position ofthe image display beam 101 when the to-be-scanned surface element 202 ismoved in a normal direction (Z-axis direction, i.e., front-to-backdirection) will be described with reference to FIGS. 6A to 6D. Thehorizontal axes of the graphs of FIGS. 6A to 6D indicate the amount ofmovement with the position of the to-be-scanned surface element 202defined by the specifications listed in Table 3 as an origin. Thevertical axes indicate the size of the diameter of the beam spot formedon the to-be-scanned surface element 202.

FIG. 6A illustrates as a comparative example a correlation between thearrival position of the image display beam 101 and the size of the beamspot diameter in the main scanning direction when the flat to-be-scannedsurface element 202 is moved in the Z-axis direction. FIG. 6B similarlyillustrates as a comparative example a correlation between the arrivalposition of the image display beam 101 and the size of the beam spotdiameter in the sub-scanning direction when the flat to-be-scannedsurface element 202 is moved in the Z-axis direction. FIG. 6Cillustrates a correlation between the arrival position of the imagedisplay beam 101 and the size of the beam spot diameter in the mainscanning direction when the longitudinally-curved to-be-scanned surfaceelement 202 is moved in the Z-axis direction. FIG. 6D illustrates acorrelation between the arrival position of the image display beam 101and the size of the beam spot diameter in the sub-scanning directionwhen the longitudinally-curved to-be-scanned surface element 202 ismoved in the Z-axis direction.

The value at the intersection of the solid line illustrated in each ofFIGS. 6A to 6D and the vertical axis of the graph is a target value ofthe beam spot diameter in Example 1. In the to-be-scanned surfaceelement 202 according to Example 1, the beam spot diameter has a targetvalue of 103 μm in the main scanning direction and a target value of 89μm in the sub-scanning direction.

The values at the intersections of two respective broken lines parallelto the vertical axis illustrated in each of FIGS. 6A to 6D and thehorizontal axis of the graph indicate the amounts of movement from theorigin. A difference in the amount of movement between the two parallelbroken lines indicates an allowable amount of movement of theto-be-scanned surface element 202 in the front-to-back direction, theallowable amount of movement being allowed to satisfy the target valueof the beam spot diameter. As illustrated in FIGS. 6A and 6B, if theto-be-scanned surface element 202 is a flat one, the allowable amount ofmovement in the main scanning direction is approximately 10.8 mm. Bycontrast, as illustrated in FIGS. 6C and 6D, the curved to-be-scannedsurface element 202 according to the present embodiment has an allowableamount of movement of approximately 13 mm.

Therefore, if the to-be-scanned surface element 202 has the curved shapecurved in the main scanning direction (longitudinal direction), theallowable amount of movement of the to-be-scanned surface element 202 inthe front-to-back direction with which the beam spot diameter in themain scanning direction has a satisfactory size can be increased.

The allowable amounts of movement of the to-be-scanned surface element202 in the sub-scanning direction are similarly compared. If theto-be-scanned surface element 202 is a flat one, the allowable amount ofmovement is approximately 7.2 mm. If the to-be-scanned surface element202 has the curved shape in the main scanning direction, the allowableamount of movement is approximately 8.5 mm. That is, the to-be-scannedsurface element 202 according to the present embodiment can increase theallowable amount of movement in the sub-scanning direction as well.

As described above, the to-be-scanned surface element 202 according tothe present embodiment has the curved shape in the main scanningdirection. This can increase the allowable value for the displacement ofthe to-be-scanned surface element 202 when the scanning optical system200 is assembled. As a result, the robustness increases.

Next, the arrival points of light on the to-be-scanned surface element202 (arrival points of the image display beam 101) will be described bycomparison like the foregoing. FIGS. 7A and 7B illustrate the arrivalpoints of light on the to-be-scanned surface element 202 according tocombinations of the angles of the micromirrors listed in Table 4. Inother words, FIGS. 7A and 7B illustrate a configuration of imageformation on the to-be-scanned surface element 202.

FIG. 7A illustrates an example where the to-be-scanned surface 202 has aflat shape. FIG. 7B illustrates an example where the to-be-scannedsurface 202 is longitudinally curved. It is shown that if theto-be-scanned surface element 202 has the shape of the curved surfacecurved in the longitudinal direction (main scanning direction), thedistortion of the image (intermediate image) can be made smaller thanwith a flat surface. Since the image distortion of the magnified virtualimage 12 displayed by the virtual image optical system 300 arranged inthe subsequent stage of the scanning optical system 200 can be reducedas well, a magnified virtual image 12 of high image quality can bedisplayed.

EXAMPLE 2

Next, another embodiment of the scanning optical system 200 will bedescribed. Table 5 below lists an example of the specifications of theHUD 1000 according to Example 2 and the dimensions of the to-be-scannedsurface element 202 which is the image forming unit.

TABLE 5 VIRTUAL IMAGE POSITION (DISTANCE 2 m FROM OBSERVER 11 TOMAGNIFIED VIRTUAL IMAGE 12) ANGLE OF VIEW (ANGLE AT WHICH 6° × 2.5°OBSERVER 11 VIEWS MAGNIFIED VIRTUAL IMAGE 12) SIZE OF IMAGE FORMING UNIT(TO-BE- 30 mm × 14.3 mm SCANNED SURFACE ELEMENT 202)

Table 6 below lists an example of the specifications of thetwo-dimensional deflection element 201 (MEMS) which is the opticalscanning unit. The tilt angle is of the micromirrors included in thetwo-dimensional deflection element 201.

TABLE 6 MAIN SCANNING SUB-SCANNING DIRECTION DIRECTION TILT ANGLE (°)16.7 9.3 EFFECTIVE SIZE (mm) 0.75 0.98

Table 7 below lists another example of the data related to the opticalelements included in the light source device 100 and the scanningoptical system 200. “Surface number” in Table 7 is assigned with thelight emitting points of the first, second, and third light sources 110,120, and 130 included in the light source device 100 as the “zerothsurface”. The light incident side of the to-be-scanned surface element202 is the ninth surface. The first to eighth surfaces refer to surfacesthat give an optical effect to the light emitted from the light sourceunits. Examples of the optical effect include convergence anddivergence.

TABLE 7 Y RADIUS OF X RADIUS OF SURFACE SURFACE CURVATURE Ry CURVATURERx DISTANCE NUMBER (mm) (mm) (mm) MATERIAL REMARKS 0 1.00E+18 1.00E+182.46 LIGHT EMITTING POINTS OF 1ST, 2ND, AND 3RD LIGHT SOURCE UNITS 110,120, AND 130 1 1.00E+18 1.00E+18 0.25 SBSL7 2 1.00E+18 1.00E+18 5.26 31.00E+18 1.00E+18 5.00 SBSL7 1ST COLLIMATOR LENS 111 2ND COLLIMATOR LENS121 3RD COLLIMATOR LENS 131 4 −6.428  −6.428 0.20 5 1.00E+18 1.00E+1830.04 1ST APERTURE 112 2ND APERTURE 122 3RD APERTURE 132 6 4.209  4.2093.00 LLAM60 CONDENSER LENS 150 7 3.026  3.026 53.79 8 1.00E+18 1.00E+18−59.57 TWO-DIMENSIONAL DEFLECTION ELEMENT 201 9 1.00E+18 58.821 0.00TO-BE-SCANNED SURFACE ELEMENT 202

The fifth surface listed in Table 7 corresponds to the aperture surfacesof the respective first, second, and third apertures 112, 122, and 132.The opening areas of the apertures are defined by the sizes of theaperture surfaces in the main scanning direction and the sub-scanningdirection. The apertures have opening areas of different sizes.

The aperture surface of the first aperture 112 corresponding to the redlight source has a size of 2.08 mm in the main scanning direction and3.04 mm in the sub-scanning direction. The aperture surfaces of thesecond aperture 122 corresponding to the green light source and thethird aperture 123 corresponding to the blue light source both have asize of 2.0 mm in the main scanning direction and 2.4 mm in thesub-scanning direction. That is, the first aperture 112 corresponding tothe first light source unit which is the red light source has an openingarea greater than that of both the second and third apertures 122 and132. This can increase the input efficiency of light from the red lightsource.

As listed in Table 7, the to-be-scanned surface element 202 has a Yradius of curvature Ry (1.00E+18) and an X radius of curvature Rx(58.821) which have a relationship of |Rx|<|Ry|. The Y radius ofcurvature Ry of the to-be-scanned surface element 22 refers to theradius of curvature at the center of the to-be-scanned surface element202 in the transverse direction (Y direction). The X radius of curvatureRx of the to-be-scanned surface element 202 refers to the radius ofcurvature at the center of the to-be-scanned surface element 202 in thelongitudinal direction (X direction).

The radius of curvature at the center of the to-be-scanned surfaceelement 202 in the transverse direction (Y radius of curvature Ry) is1.00E+18, which means a substantially flat surface. The radius ofcurvature at the center of the to-be-scanned surface element 202 in thelongitudinal direction (X radius of curvature Rx) is 58.821. Theto-be-scanned surface element 202 thus has a longitudinally-curvedcylindrical shape.

As listed in Table 7, the condenser lens 150 is an optical element thatguides converging light to the two-dimensional deflection element 201which is the optical scanning unit. The condenser lens 150 has a focallength f of greater than 150 mm with respect to light having awavelength of 587.56 nm. This can reduce the beam spot diameter on theto-be-scanned surface element 202 to form an intermediate image havinghigh resolution.

Next, the optical performance of the HUD 1000 including the scanningoptical system 200 according to Example 2 will be described. FIG. 8Aillustrates an example of an optical path diagram of the to-be-scannedsurface element 202 in the direction of a main scanning section. In thefollowing description of Example 2, the optical performance of the HUD1000 will be described by using a case where the to-be-scanned surfaceelement 202 is assumed to have a flat shape as illustrated in FIG. 8A asa comparative example. The following specifications related to theoptical performance were calculated by calculator simulation.

FIG. 8A is an optical path diagram of the comparative example in whichthe to-be-scanned surface element 202 is assumed to have a flat shape.FIG. 8B is an optical path diagram of the to-be-scanned surface element202 according to Example 2.

Next, a correlation between an arrival position of the image displaybeam 101 on the to-be-scanned surface element 202 according to Example 2and the size of the beam spot diameter will be described with referenceto FIGS. 9A and 9B. The horizontal axes of the graphs of FIGS. 9A and 9Bindicate the arrival position of light on the to-be-scanned surfaceelement 202 when the micromirrors of the two-dimensional deflectionelement 202 are set to the angles in the main scanning direction and thesub-scanning direction corresponding to “number” in the following Table8. The angles of Table 8 indicate those of the micromirrors with respectto the optical axis of the scanning optical system 200. The verticalaxes of the graphs of FIGS. 9A and 9B indicate the size of the beam spotdiameter.

FIG. 9A illustrates the graph of the comparative example where theto-be-scanned surface element 202 is assumed to be flat. FIG. 9Billustrates the graph of the to-be-scanned surface element 202. As isclear from a comparison between the graphs of FIGS. 9A and 9B, if theto-be-scanned surface element 202 has the longitudinally curved shape,the size of the beam spot diameter at each position on the to-be-scannedsurface element 202 varies less than when the to-be-scanned surfaceelement 202 is a flat one.

TABLE 8 MAIN SCANNING SUB-SCANNING NUMBER DIRECTION (°) DIRECTION (°) 17.8 −10.2 2 5.3 −10.2 3 2.7 −10.2 4 0.0 −10.2 5 −2.7 −10.2 6 −5.3 −10.27 −7.8 −10.2 8 7.8 −13.7 9 5.3 −13.7 10 2.7 −13.7 11 0.0 −13.7 12 −2.7−13.7 13 −5.3 −13.7 14 −7.8 −13.7 15 7.8 −17.2 16 5.3 −17.2 17 2.7 −17.218 0.0 −17.2 19 −2.7 −17.2 20 −5.3 −17.2 21 −7.8 −17.2 22 7.8 −11.8 235.3 −11.8 24 2.7 −11.8 25 0.0 −11.8 26 −2.7 −11.8 27 −5.3 −11.8 28 −7.8−11.8 29 7.8 −15.6 30 5.3 −15.6 31 2.7 −15.6 32 0.0 −15.6 33 −2.7 −15.634 −5.3 −15.6 35 −7.8 −15.6

Next, variations of the beam spot diameter in each arrival position ofthe image display beam 101 when the to-be-scanned surface element 202 ismoved in the normal direction (Z-axis direction, i.e., front-to-backdirection) will be described with reference to FIGS. 10A to 10D. Thehorizontal axes of the graphs of FIGS. 10A to 10D indicate the amount ofmovement with the position of the to-be-scanned surface element 202defined by the specifications listed in Table 7 as an origin. Thevertical axes indicate the size of the diameter of the beam spot formedon the to-be-scanned surface element 202.

FIG. 10A illustrates as a comparative example a correlation between thearrival position of the image display beam 101 and the size of the beamspot diameter in the main scanning direction when the flat to-be-scannedsurface element 202 is moved in the Z-axis direction. FIG. 10B similarlyillustrates as a comparative example a correlation between the arrivalposition of the image display beam 101 and the size of the beam spotdiameter in the sub-scanning direction when the flat to-be-scannedsurface element 202 is moved in the Z-axis direction. FIG. 10Cillustrates a correlation between the arrival position of the imagedisplay beam 101 and the size of the beam spot diameter in the mainscanning direction when the longitudinally-curved to-be-scanned surfaceelement 202 is moved in the Z-axis direction. FIG. 10D illustrates acorrelation between the arrival position of the image display beam 101and the size of the beam spot diameter in the sub-scanning directionwhen the longitudinally-curved to-be-scanned surface element 202 ismoved in the Z-axis direction.

The value at the intersection of the solid line illustrated in each ofFIGS. 10A to 10D and the vertical axis of the graph is a target value ofthe beam spot diameter in Example 2. The beam spot diameter of theto-be-scanned surface element 202 according to Example 2 has a targetvalue of 110 μm in the main scanning direction and a target value of 95μm in the sub-scanning direction.

The values at the intersections of two respective broken lines parallelto the vertical axis illustrated in each of FIGS. 10A to 10D and thehorizontal axis of the graph indicate the amounts of movement from theorigin. A difference in the amount of movement between the two parallelbroken lines indicates an allowable amount of movement of theto-be-scanned surface element 202 in the front-to-back direction, theallowable amount of movement being allowed to satisfy the target valueof the beam spot diameter. As illustrated in FIGS. 10A an 10B, if theto-be-scanned surface element 202 is a flat one, the allowable amount ofmovement in the main scanning direction is approximately 13 mm. Bycontrast, as illustrated in FIGS. 10C an 10D, the curved to-be-scannedsurface element 202 according to the present embodiment has an allowableamount of movement of approximately 15 mm.

Therefore, if the to-be-scanned surface element 202 has the curved shapecurved in the main scanning direction (longitudinal direction), theallowable amount of movement of the to-be-scanned surface element 202 inthe front-to-back direction with which the beam spot diameter in themain scanning direction has a satisfactory size can be increased.

The allowable amounts of movement of the to-be-scanned surface element202 in the sub-scanning direction are similarly compared. If theto-be-scanned surface element 202 is a flat one, the allowable amount ofmovement is approximately 7.2 mm. If the to-be-scanned surface element202 has the curved shape in the main scanning direction, the allowableamount of movement is approximately 9.6 mm. That is, the to-be-scannedsurface element 202 according to the present embodiment can increase theallowable amount of movement in the sub-scanning direction as well.

As described above, the to-be-scanned surface element 202 according tothe present embodiment has the curved shape in the main scanningdirection. This can increase the allowable value for the displacement ofthe to-be-scanned surface element 202 when the scanning optical system200 is assembled. As a result, the robustness increases.

Next, the arrival points of light on the to-be-scanned surface element202 (arrival points of the image display beam 101) will be described bycomparison like the foregoing. FIGS. 11A and 11B illustrate the arrivalpoints of light on the to-be-scanned surface element 202 according tocombinations of the angles of the micromirrors listed in Table 8. Inother words, FIGS. 11A and 11B illustrate a configuration of imageformation on the to-be-scanned surface element 202.

FIG. 11A illustrates an example where the to-be-scanned surface 202 hasa flat shape. FIG. 11B illustrates an example where the to-be-scannedsurface 202 is longitudinally curved. It is shown that if theto-be-scanned surface element 202 has the shape of the curved surfacecurved in the longitudinal direction (main scanning direction), thedistortion of the image (intermediate image) can be made smaller thanwith a flat surface. Since the image distortion of the magnified virtualimage 12 displayed by the virtual image optical system 300 arranged inthe subsequent stage of the scanning optical system 200 can be reducedas well, a magnified virtual image 12 of high image quality can bedisplayed.

According to the HUD 1000 described above, the to-be-scanned surfaceelement 202 is shaped to curve with its convex surface toward themagnifying concave mirror 301 of the virtual image optical system 300.This can suppress variations in the beam spot diameter on theto-be-scanned surface element 202. As a result, the amount of positionaladjustment to the to-be-scanned surface element 202 can be increased tocontrol the beam spot diameter within a target range. In other words,the allowable amount of movement of the to-be-scanned surface element202 can be increased to control the beam spot diameter within the targetrange.

In addition, distortion of the image on the to-be-scanned surfaceelement 202 can be improved. Since an optical element having power doesnot need to be arranged between the two-dimensional deflection element201 and the to-be-scanned surface element 202, a small-sized imagedisplay apparatus having high robustness and simple configuration can beprovided at low cost.

Curving the image forming unit toward the reflecting mirror can suppressvariations in the beam spot diameter on the image forming unit. Theallowable amount of movement of the image forming unit for controllingthe beam spot diameter within a target range can be increased.Distortion of the image formed on the image forming unit can beimproved.

The radius of curvature at the center of the image forming unitcorresponding to the longitudinal direction is smaller than the radiusof curvature at the center of the image forming unit corresponding tothe transverse direction. As a result, image distortion and a drop inresolution, which are likely to occur in the longitudinal direction ofthe virtual image, can be more effectively improved.

The cylindrical shape of the image forming unit can further improvelongitudinal distortion of the image and a drop in resolution.

No optical element having a condensing or diverging effect is includedbetween the optical scanning unit and the image forming unit. As aresult, an image display apparatus having high robustness can beprovided at low cost.

The optical element that guides converging light to the optical scanningunit has a focal length f of greater than 150 mm at a wavelength of587.56 nm. The beam spot diameter on the image forming unit can thus bemade smaller to display a high-resolution image.

The aperture after the collimator lens for the red light source has anopening area greater than that of the apertures after the collimatorlenses for the green and blue light sources. This can increase the inputefficiency of light from the red light source.

Lasers and LEDs can be used as the light sources. As compared to thecase of using lamps as the light sources, small-sized and long-livedlight sources can thus be included, and an image of high colorreproducibility can be displayed.

The image display apparatus can be mounted on a moving body so that itsdriver can recognize alarms and information with less movement of theline of sight.

According to the embodiments described above, a reduction in size andcost can be achieved while maintaining the viewability (quality) of avirtual image.

Although the invention has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

What is claimed is:
 1. An image display apparatus comprising: a lightsource device including a light source unit; a scanning optical systemincluding an image forming unit on which an intermediate image is formedby light from the light source unit; and a virtual image optical systemconfigured to guide light of the intermediate image by using areflecting mirror and a curved transmissive reflection member, wherein:the scanning optical system includes an optical scanning unit configuredto two-dimensionally to scan the light from the light source unit in amain scanning direction and a sub-scanning direction perpendicular tothe main scanning direction of the image forming unit, and in an XYZthree-dimensional coordinate system where an X direction corresponds tothe main scanning direction, a Y direction corresponds to thesub-scanning direction, and a Z direction is perpendicular to both the Xdirection and the Y direction, the image forming unit is a transmissivemember curved in the X direction of the curved transmissive reflectionmember.
 2. The image display apparatus according to claim 1, wherein|Rx|<|Ry| is satisfied, where Rx is a radius of curvature at a center ofthe image forming unit corresponding to the longitudinal direction whenthe intermediate image is displayed on the curved transmissivereflection member, and Ry is a radius of curvature at the center of theimage forming unit corresponding to a transverse direction when theintermediate image is displayed on the curved transmissive reflectionmember.
 3. The image display apparatus according to claim 1, wherein, inan XY plane of the XYZ three-dimensional coordinate system, an opticallength of light incident on one end of the reflecting mirror from theoptical scanning unit through one end of the image forming unit isdifferent from an optical length of light incident on another end of thereflecting mirror from the optical scanning unit through an opposite endof the image forming unit.
 4. The image display apparatus according toclaim 1, wherein the scanning optical system does not include an opticalelement having a condensing effect or an optical element having adiverging effect between the optical scanning unit and the image formingunit.
 5. The image display apparatus according to claim 1, wherein thelight source device includes a condensing element for converging lighton the optical scanning unit, the condensing element having a focallength f of greater than 150 mm with respect to light having awavelength of 587.56 nm.
 6. The image display apparatus according toclaim 1, wherein: the light source unit includes independent lightsources corresponding to red, green, and blue, respectively, andcollimator lenses and apertures corresponding to the respective lightsources, the collimator lenses and apertures making parallel lightemitted from the respective light sources; and the aperturecorresponding to the red light source has an opening area greater thanboth that of the aperture corresponding to the green light source andthat of the aperture corresponding to the blue light source.
 7. Theimage display apparatus according to claim 1, wherein the light sourceunit is a laser or an LED.
 8. The image display apparatus according toclaim 1, wherein the curved transmissive reflection member is a frontwindshield of a moving body.